1. Field of the Invention
The present invention concerns amplifier circuits for infrared detectors and more particularly amplifier circuits suitable for fabrication along with a detector array on a single large-scale integrated structure.
2. Background of the Invention
A number of applications have been developed for infrared surveillance systems employing a cryogenically cooled photovoltaic infrared detector array arranged on a large-scale integrated structure. In some of these applications, the source under surveillance may have a very low infrared photon output differing only slightly from the background infrared radiation level. Typical of such applications are space-based sensing of regional agricultural conditions and absolute radiometric measurements.
Usually each infrared detector element in the array is provided with an individual amplifier circuit, preferably with both the detector array and the dedicated amplifiers fabricated on a single large-scale integrated structure. Due to the environment in which the infrared survey system operates, it is generally desirable to minimize power requirements and heat dissipation.
Photovoltaic infrared detectors have conventionally been amplified by discrete component transimpedance-type amplifier circuits, such as shown in FIG. 1, typically employing a discrete amplifier variably biasing the detector. Discrete component transimpedance amplifiers, however, are not compatible with large-scale integrated technology and generally require unacceptable levels of power to operate.
Another conventional detector amplifier circuit is the so-called synthetic transimpedance amplifier circuit, an example of which is shown in FIG. 2. In this circuit the detector is coupled to both a buffer FET and an amplifier with the output of the amplifier coupled to the gate of the buffer FET. The currnnt produced by the detector is directly injected into a storage capacitor with the potential across the capacitor periodically sampled by an external circuit. Thus, the current output of the detector is converted into a voltage which is in part related to the size of the storage capacitor. A reset FET, coupled to a reference voltage and controlled by a clock pulse, periodically resets the storage capacitor. A source follower amplifier is used to buffer the storage capacitor from the external circuit.
Synthetic transimpedance amplifiers, while compatible with large-scale integrated array applications, still suffered from a number of disadvantages. Typically the bias across the detector was determined by a bias across the buffer FET gate. This precluded optimally biasing the detector, reduced the uniformity of detector biasing, and increased apparent detector noise. The input impedance of the synthetic transimpedance amplifier is equal to the voltage change or detector input required to increase the current by e.sup.1 divided by the detector current. Thus, for small detector currents, the synthetic transimpedance amplifier input impedance increased, resulting in a decreased injection efficiency.
Consequently, there presently exists a need for a photovoltaic infrared detector amplifier circuit which is suitable for large-scale integrated structure fabrication, provides for independent optimal biasing of the detector array, and has an increased injection efficiency.